1. Field of the Invention
The invention concerns a method of and a circuit arrangement for improving the transmission properties of an echo-affected transmission link in a telecommunications network, which comprises for example interconnecting the analog and digital circuits of telecommunication systems. This interconnection creates problems with the transition of two-wire transmission lines, such as are typical for the subscriber connection lines in analog telecommunications networks, to four-wire transmission lines in digital telecommunications networks. These two-wire/four-wire transitions are provided for by a hybrid circuit, where mismatches in the balancing line with respect to the current line properties can never be fully avoided, creating signal reflections in the form of interfering echoes which can considerably reduce speech intelligibility between the subscribers, or which can significantly increase the error rate during data transmission.
2. Description of Related Art
It is basically known to eliminate the effect of echoes with echo compensators, see R. Wehrmann et. al: "Signal processing method for improving speech communication via hands-free facilities", The Telecommunications Engineer, 40th. year, October 1994, pages 27-28. This essay describes how the transmit function of the real system can be fairly accurately simulated with an echo compensator. The signal arriving at the subscriber then passes through both the real system as well as the one simulated by the echo compensator, and after that the output signal of the echo compensator is subtracted from the echo-affected signal of the real system, thus compensating for most of the echo. In a real system, for example loudspeaker-room-microphone, such an echo compensator can considerably reduce the so-called local echo which is created by the acoustic coupling between the loudspeaker and the microphone.
Mismatched hybrid circuits in telecommunications networks with two-wire/four-wire transitions create an electrical coupling between the transmit path 1.3 and the receive path 1.5, and a portion of the transmitted signal is reflected back to the subscriber on the receive path as a so-called line echo. Echoes of different intensities and duration, which can be roughly divided into near and distant echoes, are a function of the distance between the subscriber and the hybrid circuit along the transmit path, and different transmission means and transmission devices such as for example satellite links, submarine links, encoding devices or highly compressed audio-video-codecs. A line transmission system with a transmit path and a receive path therefore differs significantly from a loudspeaker-room-microphone, particularly with regard to different echo delay times and different line attenuations.
Adaptive digital filters are preferably used as echo compensators, whose filter coefficients can be determined and adjusted for example in accordance with the Normalized Least Mean Square Algorithm, briefly called NLMS algorithm, see T. Huhn, H.-J. Jentschel: "Combination of noise reduction and echo compensation during hands-free operation", Communications Electronics, Berlin 43 (1993), pages 274-280. The filter coefficients must be constantly updated in accordance with the changing echo signal. The required filter length is determined by the sampling frequency and the longest echo delay time. The delay time from distant echoes is very different and can be over 250 ms, so that either the number of filter coefficients to be calculated becomes very large, or a method to determine the individual echo delay times must be used, for example a correlation analysis which requires a large computation effort, see U.S. Pat. No. 5,467,394. From this patent it is known to perform a correlation measurement between the received echo signal and the transmitted signal to ascertain the precise instant for determining the echo delay times and the filter coefficients. However any practical application of this solution fails because its cost is too high.
It is known to use companders to reduce the effects of local echoes during hands-free operation, whereby transmit signals above a reference value are compressed to a uniform signal level, they are then amplified if they have the same reference value, and are attenuated if they are below the reference value. In this way a circuit arrangement for the dynamic control of a terminal is known from the specification U.S. Pat. No. 4,891,837, whereby a controllable compander adapts the degree of amplification of the signal voltages produced by a microphone to the reference value, for the purpose of hands-free operation. The result is that fluctuations in the microphone signal, which are created by moderate variations in the talking distance and the individual speech volume, are compressed to a more uniform loudness level at the reproduction end.
Furthermore a method of improving the transmission properties of an electro-acoustical facility is known, whereby the position of the compander characteristic transfer function is automatically controlled, see EPO Publication 0 600 164 A1 and its German parent having Application No. DE 42 29 912 A1. The method detects whether the transmitted signal is generated by speech or by noise. The magnitude of the control signal for shifting the position of the compander characteristic, which is produced with the speech and noise detection, achieves the result that the speech being transmitted is sent at a constant level, that the background noise level is lowered for the transmission, and that the volume of the received reproduction is increased with increasing ambient noise.
The use of the previously described compander arrangement to suppress local echoes in hands-free facilities is known. However this compander arrangement is not suitable for suppressing line echoes, because with lines it must be taken into consideration that the signal level in the lines of telecommunication networks, particularly in dialing lines, can vary from one connection to another in the range of greater than 20 dB, and that the signal delay times can vary for example within the range of 10 ms to 10 s as a function of the number of satellite links between the connection points, or the kind of signal processing systems, for example by compression.